The present invention relates generally to variable-directivity microphone devices, and more particularly to a variable-directivity microphone device in which the phase of the high-frequency range component of the output signal of one microphone of at least two microphones is as a result invented and this high-frequency range component is mixed to the output signal of the other microphone.
Heretofore, as a microphone device capable of varying its directivity, there has been a microphone device in which two microphones having primary sound-pressure gradient unidirectivity (hereinafter referred to as primary unidirectivity) are arranged in a mutually confronting state, and their outputs are mixed by means of a mixer. Furthermore, there has also been a microphone device in which two unidirectional microphones are arranged to face the same direction, and the output of one of the microphones is mixed with opposite phase with the output of the other microphone.
In each of these devices, the directivity of the microphone device is varied effectively, by varying the mixture ratio to obtain the final output signal.
In this case, the directional pattern P obtained by mixing the outputs of the first and second microphones, in terms of the sensitivity A of the first microphone of the two microphones, the sensitivity B of the second microphone, the angle .theta. between the axis l of both microphones and the sound source, the distance D between the first and second microphones, and the wavelength constant K, becomes as follows. ##EQU1## When the sensitivities A and B of the first and second microphones are identical, that is, A=B, the above Eq. (1) becomes ##EQU2## By appropriately selecting the value of A in Eq. (2), a directional pattern of secondary unidirectivity can be obtained.
In this known device, however, since the outputs of the two microphones are mixed with mutually opposite phases, a dip in the frequency characteristic occurs at a frequency F corresponding to the wavelength of the picked-up sound wave when this wavelength is equal to the distance D between the front faces of the two microphones (F being 11.3 KHz, for example, when D is 3 cm.). At the same time, at a frequency where the wavelength of the sound wave is very much less than the distance D, a frequency characteristic wherein the response decreases in a proportion of 6 dB/oct with decreasing frequency is exhibited.
Accordingly, in a known microphone device, the output of the aforementioned mixer is passed through an equalizer having a characteristic which is the opposite of the above described frequency characteristic, that is, a frequency characteristic wherein the response increases with decreasing frequency. By this expedient, a signal of flat characteristic wherein the frequency characteristic has been corrected, particularly in the medium-and low-frequency ranges, is obtained from the output of the equalizer.
In a signal obtained from the above mentioned mixer, however, the response decrease in the frequency characteristics is of the order of 29 dB at 100 Hz, for example, the above mentioned equalizer must have an equalizing characteristic which carries out response correction of the order of 29 dB at 100 Hz. Consequently, for the above mentioned equalizer, an equalizer having an equalizing characteristic of great correction quantity must be used. As a result, the S/N ratio of the signal obtained from the equalizer is small, particularly in the low-frequency range. Furthermore, in the case where the microphones are used outdoors, noise due to wind in a range of relatively low-frequency is easily produced. Furthermore, the problem is that touch noise and the like in a range of relatively low-frequency is also easily produced when the microphones are touched.